Attention has been drawn to a technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface. These transistors are widely used for electronic devices such as an integrated circuit (IC) and an image display device (also simply referred to as a display device). Although a silicon-based semiconductor material is a common material for a semiconductor thin film applicable to a transistor, an oxide semiconductor has been attracting attention as another material.
For example, a technique for forming a transistor using an oxide semiconductor such as zinc oxide or an In—Ga—Zn-based oxide semiconductor is disclosed (see Patent Documents 1 and 2).
Another technique is also disclosed: oxide semiconductor layers with different electron affinities (or lower levels of the conduction bands) are stacked to increase the carrier mobility of a transistor (see Patent Documents 3 and 4).